Re: [Qemu-devel] [RFC PATCH] gdbstub: Avoid NULL dereference in gdb_handle_packet()

[Luc Michel]

On 1/18/19 12:22 PM, Philippe Mathieu-Daudé wrote:
> The "Hg" GDB packet is used to select the current thread, and can fail.
> GDB doesn't not check for failure and emits further packets that can
> access and dereference s->c_cpu or s->g_cpu.
> 
> Add a check that returns "E22" (EINVAL) when those pointers are not set.
> 
> Peter Maydell reported:
>   GDB doesn't check the "Hg" packet failures and emit a
>   "qXfer:features:read" packet which accesses th
>   Looking at the protocol trace from the gdb end:
>   (gdb) set debug remote 1
>   (gdb) target remote :1234
>   Remote debugging using :1234
>   Sending packet:
>   
> $qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+;fork-events+;vfork-events+;exec-events+;vContSupported+;QThreadEvents+;no-resumed+;xmlRegisters=i386#6a...Ack
>   Packet received: PacketSize=1000;qXfer:features:read+;multiprocess+
>   Packet qSupported (supported-packets) is supported
>   Sending packet: $vMustReplyEmpty#3a...Ack
>   Packet received:
>   Sending packet: $Hgp0.0#ad...Ack
>   Packet received: E22
>   Sending packet: $qXfer:features:read:target.xml:0,ffb#79...Ack
>   [here is where QEMU crashes]
> 
>   What seems to happen is that GDB's attempt to set the
>   current thread with the "Hg" command fails because
>   gdb_get_cpu() fails, and so we return an E22 error status.
>   GDB (incorrectly) ignores this and issues a general command
>   anyway, and then QEMU crashes because we don't handle s->g_cpu
>   being NULL in this command's handling code.
> 
> Fixes: c145eeae1cc
> Reported-by: Peter Maydell <address@hidden>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
> RFC because these are many if..break but I can't think of a cleaner way
> today.
I can't think of a better way without an important refactoring of gdbstub.c.

> The protocol isn't specific about the error, can it be "E03" for ESRCH
> (No such process)?
> ---
>  gdbstub.c | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index bfc7afb509..480005b094 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -1306,6 +1306,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          put_packet(s, "OK");
>          break;
>      case '?':
> +        if (!s->c_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
>          /* TODO: Make this return the correct value for user-mode.  */
>          snprintf(buf, sizeof(buf), "T%02xthread:%s;", GDB_SIGNAL_TRAP,
>                   gdb_fmt_thread_id(s, s->c_cpu, thread_id, 
> sizeof(thread_id)));
> @@ -1394,6 +1398,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          /* Detach packet */
>          pid = 1;
>  
> +        if (!s->c_cpu || !s->g_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
Even though this is probably true (s->[gc]_cpu == NULL probably means
"no process attached"), the detach packet in itself carries the PID to
detach from. So I would go for a code hardening, something like:

         process = gdb_get_process(s, pid);
+        if (!process->attached) {
+            put_packet(s, "E22");
+            break;
+        }
         gdb_process_breakpoint_remove_all(s, process);
         process->attached = false;

-        if (pid == gdb_get_cpu_pid(s, s->c_cpu)) {
+        if (s->c_cpu && pid == gdb_get_cpu_pid(s, s->c_cpu)) {
             s->c_cpu = gdb_first_attached_cpu(s);
         }

-        if (pid == gdb_get_cpu_pid(s, s->g_cpu)) {
+        if (s->g_cpu && pid == gdb_get_cpu_pid(s, s->g_cpu)) {
             s->g_cpu = gdb_first_attached_cpu(s);
         }


>          if (s->multiprocess) {
>              unsigned long lpid;
>              if (*p != ';') {
> @@ -1429,6 +1437,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          put_packet(s, "OK");
>          break;
>      case 's':
> +        if (!s->s_cpu) {
if (!s->c_cpu) {

> +            put_packet(s, "E22");
> +            break;
> +        }
>          if (*p != '\0') {
>              addr = strtoull(p, (char **)&p, 16);
>              gdb_set_cpu_pc(s, addr);
> @@ -1441,6 +1453,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>              target_ulong ret;
>              target_ulong err;
>  
> +            if (!s->s_cpu) {
if (!s->c_cpu) {

> +                put_packet(s, "E22");
> +                break;
> +            }
>              ret = strtoull(p, (char **)&p, 16);
>              if (*p == ',') {
>                  p++;
> @@ -1463,6 +1479,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          }
>          break;
>      case 'g':
> +        if (!s->g_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
>          cpu_synchronize_state(s->g_cpu);
>          len = 0;
>          for (addr = 0; addr < s->g_cpu->gdb_num_g_regs; addr++) {
> @@ -1473,6 +1493,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          put_packet(s, buf);
>          break;
>      case 'G':
> +        if (!s->g_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
>          cpu_synchronize_state(s->g_cpu);
>          registers = mem_buf;
>          len = strlen(p) / 2;
> @@ -1485,6 +1509,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          put_packet(s, "OK");
>          break;
>      case 'm':
> +        if (!s->g_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
>          addr = strtoull(p, (char **)&p, 16);
>          if (*p == ',')
>              p++;
> @@ -1504,6 +1532,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>          }
>          break;
>      case 'M':
> +        if (!s->g_cpu) {
> +            put_packet(s, "E22");
> +            break;
> +        }
>          addr = strtoull(p, (char **)&p, 16);
>          if (*p == ',')
>              p++;
> @@ -1642,6 +1674,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>              put_packet(s, "OK");
>              break;
>          } else if (strcmp(p,"C") == 0) {
> +            if (!s->g_cpu) {
> +                put_packet(s, "E22");
> +                break;
> +            }
>              /*
>               * "Current thread" remains vague in the spec, so always return
>               * the first thread of the current process (gdb returns the
> @@ -1745,6 +1781,10 @@ static int gdb_handle_packet(GDBState *s, const char 
> *line_buf)
>              const char *xml;
>              target_ulong total_len;
>  
> +            if (!s->g_cpu) {
> +                put_packet(s, "E22");
> +                break;
> +            }
>              process = gdb_get_cpu_process(s, s->g_cpu);
>              cc = CPU_GET_CLASS(s->g_cpu);
>              if (cc->gdb_core_xml_file == NULL) {
> 

I think you are missing the check for 'p' and 'P' packets, and for the
qOffsets packet in user mode (around line 1701).

Thanks.

-- 
Luc

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